John & Bruce,
I've done some more digging. If I go to File>Open Measurement File, I get a pop-up box saying Look In: REW Cal Files and it defaults to Files of Type: .mdat. Now, the files that were generated when I calibrated my sound card are in there. And, if I ask it for Files of Type: All Files, two of the files that I recently saved are listed but if I select them and try to open them, I get a 'Load measured data set error' that says - C:\Documents and Settings\John\My Documents\REW Cal Files\FE167E SW1 Jul 15 13.mdat not found. Similar with the other file that I saved. BTW, I think this file (beginning 'FE167E') has been truncated; I may have inadvertently included a colon in the original but I would have expected an error message if that was the case when I tried to save it. So, I'm really not sure what is going on.
I don't want to re-install REW if I can avoid it because I would presumably have to re-calibrate my sound card, etc. before being able to run actual room measurements.
Sorry this is a bit heavy-going but I need your help, guys.